Starting, i.e. igniting, a gas turbine can be a complex process, but can be described generally as including the following steps. First, the turbine is ventilated so as to remove any fuel that may still be in the combustion chamber, the turbine assembly, or in the turbine piping. Then the turbine is rotated until a desired compression is reached, at which point, the turbine fuel is introduced and ignited. After the ignition, load is connected. One of the reasons for ventilating before the turbine is rotated is to prevent unwanted ignition or combustion before the desired compression is reached.
Improper ignition can lead to delay in reaching proper operating levels and also can lead to damages. Even worse, if too much fuel is delivered and present, there is a risk of explosion. Whether there is too much or not enough fuel depends, at least in part, on the heat value or flammability of the fuel, and fuel-air mixture properties. A fuel-to-air ratio for one fuel may be too much due to the high flammability of the fuel, while the same fuel-to-air ratio for a low flammability fuel may not be enough for ignition.
Due to market fluctuations, it is often beneficial for an operator to operate the gas turbine using different types of turbine fuels. Thus, the fuel gas characteristics may not be known when the gas turbine is to be turned on for operation. When the fuel gas characteristics are not known at the time of the turbine ignition, a conservative approach is taken to ignite the turbine for safety reasons.
In the conservative approach, multiple ignition attempts are the norm. Initially, a very low fuel-to-air mixture is provided to minimize risk of explosion and the ignition is attempted. If the attempt is unsuccessful, the turbine system is ventilated. Then the parameters are changed such as increasing the fuel-to-air ratio, increasing the turbine speed, changing fuel and/or air, or flow, and so on and another ignition attempt is made. This process is repeated until the turbine is successfully ignited.
The process followed to ignite the turbine when the fuel gas characteristics are unknown can be generally described as assuming the worst case, and incrementally changing the ignition parameters until the successful ignition occurs. Unfortunately, multiple ignition attempts take time, i.e. causes delay. Also, multiple attempts increase wear and tear on the turbine.
On the other hand, if the fuel gas composition is known, then the number of ignition attempts can be reduced, down to a single attempt in some instances. For example, the fuel gas supplier may provide this information. When the fuel gas composition is known, then the ignition parameters may be determined more precisely. Reducing the number of ignition attempts enables the turbine to reach operation levels quicker and also reduces wear and tear. However, it is not always possible for the supplier to provide the information.
Another way is to include sensing devices to measure the fuel gas characteristics. In the US Patent Publication 2008/0115482 issued to LaGrow et al., a gas turbine engine that includes an integrated fuel characterization system is disclosed. The integrated fuel gas characterization system determines the amount of energy provided by the fuel prior to combustion of the fuel in the combustion stage. According to LaGrow et al., the integrated fuel gas characterization system provides improved turbine engine start-up reliability by tuning the turbine engine operating parameters using fuel gas energy content measurements obtained prior to actual start-up.
In LaGrow et al., either a Wobbe meter or a gas chromatograph is provided upstream of the turbine's combustion chamber. In this way, the Wobbe index or the fuel gas composition can be measured before the fuel gas is combusted in the gas turbine. Based on the measured Wobbe index or the fuel gas composition, the turbine's operating parameters are tuned so that by the time the measured gas enters the combustion chamber, proper adjustments are made. To allow time for sufficient measurements to take place and operating parameters be adjusted, a buffer tank is provided.
Sensing devices such as the Wobbe meter, gas chromatograph, and calorimeter allow for faster ignition by reducing the number of attempts, but these types of sensing devices are expensive. Adding to the cost is that often, the sensors are repeated for redundancy so as to provide some margin of safety. For existing turbine systems that are without these sensors, upgrading these systems with the sensors can represent a significant cost increase and service disruption. Another disadvantage is that the sensing devices may not always be reliable.
Thus, it is desirable to reduce the number of gas turbine ignition attempts even when the fuel gas characteristics are unknown without requiring such sensing devices.